Intraluminal prostheses used to maintain, open, or dilate blood vessels are commonly known as stents. Stent constructions generally include lattice type cylindrical frames that define a plurality of openings. Common frameworks for stents include, for example, individual rings linked along the length of the stent by a linking member, a continuous helically wrapped member (that may include one or more linking members), a braid or a mesh formed into a tubular structure, and a series of interconnected struts. Stents may be formed by arranging one or more members in a pattern along a longitudinal axis to define essentially a cylinder and connecting the one or more members or otherwise affixing them in position (e.g., interconnecting with a filament). Stents may also be formed by cutting openings into a tube of material (e.g., shape memory alloy).
Stents are either self-expanding or balloon expandable. Self-expanding stents are delivered to a blood vessel in a collapsed condition and expand in vivo following the removal of a constraining force and/or in the presence of an elevated temperature (due to material properties thereof), whereas balloon expandable stents are generally crimped onto a balloon catheter for delivery and require the outwardly directed force of a balloon for expansion. Stents can be made of various metals and polymers and can include a combination of self-expanding and balloon expandable properties.
Synthetic vascular grafts are routinely used to restore the blood flow in patients suffering from vascular diseases. For example, prosthetic grafts made from expanded polytetrafluoroethylene (ePTFE) are commonly used and have shown favorable patency rates, meaning that depending on a given time period, the graft maintains an open lumen for the flow of blood therethrough. Grafts of ePTFE may be manufactured in a number of ways, including, for example, extrusion of a tube (seamless), extrusion of a sheet that is subsequently formed into a tube (one or more seams), helical wrapping of ePTFE tape around a mandrel (e.g., multiple seams or preferably a single helical seam), etc. Grafts can also be created from fibers woven or knitted into a generally tubular shape.
It is known in the art to use stents in combination with vascular grafts to form stent-grafts. Because stent-grafts are often intraluminally deployed in vessels of varying sizes and tortuosity, flexibility can be an important consideration. Flexibility can be imparted to a stent-graft in a variety of ways, including, for example, connection of the stent to the one or more graft layers, configuration of the stent and/or graft layer(s), spacing of the stent struts, rings, or members along the length of the graft(s), etc. Another important consideration in the design of a stent-graft is the ability of the stent to withstand stress and fatigue, caused, for example, by plastic deformations occurring at strut junctions when the stent is subjected to circumferential forces. Stent strength can be enhanced through material choice, stent configuration, arrangement and configuration of graft layers, connecting members between stent members, etc. Another consideration in the design of certain stent-grafts is properties to resist kinking of the stent-graft. For example, when a stent-graft is positioned in a bend in a blood vessel or bypass graft, depending on the acuteness of the angle of the bend, the stent-graft can potentially kink and thereby become unsuitable for passage of blood therethrough.
The following references relate to stents and stent-grafts: U.S. Pat. No. 5,104,404 to Wolff; U.S. Pat. No. 5,507,771 to Gianturco; U.S. Pat. No. 5,556,414 to Turi; U.S. Pat. No. 6,409,754 to Smith et al.; U.S. Pat. No. 6,605,110 to Harrison; U.S. Pat. No. 6,673,103 to Golds et al.; U.S. Pat. No. 6,875,228 to Pinchasik et al.; and U.S. Pat. No. 6,911,041 to Zscheeg, each of which is incorporated by reference in its entirety into this application.
It is also known in the art to use outsert plastic injection molding to create long lasting, fatigue resistant hinges. The hinge is created from a thin section of plastic that generally connects two segments of a part to keep them together and permit the two segments to pivot, generally by opening and closing, repeatedly. The integrated hinge is very fatigue resistant because the long polymer molecules of the plastic are aligned across the hinge. Typically, such hinges are used in containers such as toolboxes, fish tackle boxes, and other high volume applications.
Generally, the hinge is created from very flexible plastic materials such as polypropylene or polyethylene. The material is chosen to permit repeated cycles of the hinge without failing. Different techniques can be used to orient the fibers across the hinge to increase the hinge strength. When molding the part, the hinge may be oriented, relative to the injection flow, so that the plastic flows across the hinge. In addition, when a part comes out of the mold, it may be flexed while it is still hot or warm to ensure that the fibers are correctly oriented. Finally, the hinge could be made by coining, which compresses the hinge to its predetermined thickness after the part has been injection molded. The strain induced is greater than the yield stress of the plastic, which plastically deforms the hinge. However, the amount of coining should be less than the ultimate stress, to keep the hinge from fracturing.
The following references relate to injection molded hinges: U.S. Pat. No. 4,518,092 to Contreras, Sr; U.S. Pat. No. 5,353,948 to Lanoue et al.; and U.S. Pat. No. 5,762,852 to Hettinga, each of which is incorporated by reference in its entirety into this application.
Applicants have recognized that it would be desirable to provide a stent or stent-graft that is axially and/or angularly flexible and able to maintain strength under high stress/fatigue environments, embodiments of which are described herein along with methods of making the same.